A Level Chem 1.7 Oxidation, Reduction and Redox Equations

Cr₂O₇^2– can oxidise SO₃^2– in acidic conditions to form Cr³⁺ and SO₄^2–

Deduce a half-equation for the oxidation of SO₃^2– to SO₄^2– (1)

SO₃^2- + H₂O → SO₄^2– + 2H⁺ + 2e^– (1)

Cr₂O₇^2– can oxidise SO₃^2– in acidic conditions to form Cr³⁺ and SO₄^2–

Half-equation for the reduction of Cr₂O₇^2– to Cr³⁺ (1)

Cr₂O₇^2– + 14H⁺ + 6e^– → 2Cr³⁺ + 7H₂O (1)

Cr₂O₇^2– can oxidise SO₃^2– in acidic conditions to form Cr³⁺ and SO₄^2–

Deduce the overall equation for the oxidation of SO₃^2– by Cr₂O₇^2– (1)

3SO₃^2- +Cr₂O₇^2– + 8H⁺ → 3SO₄ ^2– + 2Cr³⁺ + 4H₂O(1)

Which of these is a redox reaction?

A CaO + SiO₂ ⟶ CaSiO₃

B H₂SO₄ + Na₂O ⟶ Na₂SO₄ + H₂O

C NaBr + H₂SO4 ⟶ NaHSO₄ + HBr

D Mg + S ⟶ MgS (1)

D (1)

• Magnesium goes from 0 to +2 → oxidation

• Sulfur goes from 0 to −2 → reduction

Which of these species is the best reducing agent?

A Cl₂

B Cl⁻

C I₂

D I⁻ (1)

D (1)

I⁻ loses an electron more easily than Cl⁻

In which reaction is hydrogen acting as an oxidising agent?

A Cl₂ + H₂⟶ 2HCl

B (CH₃)₂CO + H₂⟶ (CH₃)₂CHOH

C N₂ + 3H₂⟶ 2NH₃

D 2Na + H₂⟶ 2NaH (1)

D (1)

Sodium goes from 0 → +1 (oxidised)

Hydrogen goes from 0 → −1 (reduced)

The reaction of calcium with water is a redox reaction. Explain, in terms of oxidation states, why this reaction involves both oxidation and reduction. (2)

Oxidation state of Ca from 0 to +2, so Ca is oxidised (1)

Oxidation state of H from +1 to 0, so H is reduced (1)

In acidic solution, IO^3– ions oxidise iodide ions to iodine.

IO₃^– + 5 I^– + 6 H⁺ → 3I₂ + 3 H₂O

Give a half-equation for the oxidation of iodide ions to iodine. Deduce the half-equation to show the reduction process in this reaction.

Oxidation half-equation ___________________________________________________________

Reduction half-equation

___________________________________________________________ (2)

Oxidation half equation 2I^– → I₂ + 2e^– (1)

Reduction half equation 2 IO₃ – + 12 H⁺ + 10 e^– → I₂ + 6 H₂O (1)